The present invention relates to a method of measuring the intensity of radiation transmitted through a body.
The present invention may be used to particular advantage for measuring the intensity of radiation transmitted through articles of fibrous material, and in particular in the tobacco industry for measuring the intensity of radiation transmitted through cylindrical tobacco products comprising a continuous rod of shredded tobacco, a continuous filter rod, cigarettes or filters.
The above measurement is normally made on cigarette manufacturing and transfer machines for controlling the density of the tobacco product and detecting any gaps, lumps or foreign bodies in the tobacco.
The intensity of radiation transmitted through a cylindrical body is normally measured by directing a beam of radiation of intensity I and substantially constant distribution onto the cylindrical body in a direction substantially perpendicular to the longitudinal axis of the body; focusing the transmitted beam; determining the intensity It transmitted through the fibrous material; and comparing the detected values with given threshold values.
As incident energy intensity I is related to transmitted energy intensity It according to the equation: EQU It=I*e.sup.-ad
where "d" is the length of the optical path in the material through which the radiation travels, i.e. the thickness of the body at the point through which the radiation travels, and "a" is an attenuation constant characteristic of the density of the material and the material itself, the transmitted energy intensity contributed by the thinner portions, i.e. with a short optical path length "d", is much greater than that contributed by the central portions. This is even more noticeable when the material of which the body is made is of a discontinuous nature, e.g. fibrous, as in the case of tobacco products.
Moreover, as focusing the output beam is equivalent to adding the various intensity contributions, the intensity contributed by the central portions is concealed by that of the thinner portions.
The above drawbacks are particularly evident when using radiation in the infrared spectrum, which is nevertheless preferable to other types of radiation in terms of operator safety.
One known solution to the problem is to direct the beam onto the central portion only of the cylindrical body, where the optical paths of the beam through the body are substantially of the same length, so that, saving any gaps or foreign bodies in the body, the intensity contribution values are all of the same order of magnitude.
The above measuring method, however, fails to provide for complete control by excluding important portions of the body.